This invention relates to a position encoder which produces a digital indication of the position of a movable mechanical member.
Typically, an electronic transmission control system for a multiple gear ratio vehicle transmission will include a multiposition gear selector lever and a linear and rotary encoder which supplies a signal or code representing the position of the lever to a electronic control unit. The encoder will be connected to the control unit by wires or circuits. It is well known that linear and rotary encoders use so called Gray codes to avoid ambiguous readings as the bits change between digital words. Gray codes (which may have redundant bits) change only one bit at a time between adjacent positions. That is, words relating to adjacent positions are identical except for one bit. However, a fault in the wires or circuit can change one or more bits of the digital word received by the control unit. It can be expected that a typical fault might affect only a single bit. However, with previously proposed Gray codes even a change in only one bit could cause the digital word received by the control unit to change to a word which would represent a large change in the position of the gear selector lever. This could be dangerous in a transmission system with multiple forward and reverse gear ratios. For example, in a transmission system with eight forward gear ratios and four reverse gear ratios, if the gear selector was in a forward high gear ratio position, it would be dangerous if an error in one bit changed the digital word to a word which represented a reverse gear ratio.